just thought I'd point out that the challenge doesn't require unique challenges...you can repeat them until you clear all the bonus goals

Although you can repeat the Rail Grinds and, if you successfully collect all the gold nuggets, will receive a chest deposited in to your Imagination Vault, the Bonus Goal "Grinder" found inside the Academy does, indeed, require unique Rail Grinds.

Has anyone found any other Rail Grinds in the other Elemental Sensei zones besides Dark and Magic? If not, then here's hoping the Enchanted Elven Forest has them ... or else Toys for Bob screwed up the Bonus Goal and, like Drawdler pointed out, they were not suppose to be "unique" Rail Grinds to count towards the Goal but are coded incorrectly making the Goal (and thus 100%'ing the game) impossible :-(